Microvalves for microfluidic devices can be made that comprise generally a “cast-in-place” mobile monolithic polymer element disposed within a microchannel and driven by a displacing force that can be fluid (either liquid or gas) pressure or an electric field to provide for control of fluid flow. As a means for controlling fluid flow, these microvalves possess the additional advantage that they can be used to effect pressure and electric field driven flows, eliminate or enhance diffusive or convective mixing, inject fixed quantities of fluid, and selectively divert flow from one channel to various other channels. They can also be used to isolate electric fields and, as a consequence, locally isolate electroosmotic or electrophoretic flows. The mobile polymer monolith microvalve architectures embodied in U.S. patent application Ser. Nos. 09/695,816 and 10/141,906 entitled “Mobile Monolithic Polymer Elements for Flow Control in Microfluidic Devices” filed Oct. 24, 2000 and May 8, 2002; and Ser. No. 10/245,224 entitled “Fluorinated Silica Microchannel Surfaces” filed Sep. 16, 2002, incorporated herein by reference in their entirety, hold great potential for sophisticated fluid routing on a variety of microfluidic platforms, including those used for chemical analysis, biosensors, toxin detection, chemical separation, combinatorial chemical synthesis, and protein crystallization. In particular, these architectures are unique in their ability to hold off solvents and pressures appropriate for miniaturized HPLC separations.
Although the above-referenced U.S. patent applications disclose and claim monolithic polymer valve architectures designed to control fluid and current flow, the phase-separated polymerization techniques used to prepare the polymer monolithic structures described therein have certain limitations. The ability of polymer microvalve architectures to control fluid flow is dependent on polymer formulations that, when polymerized, have the following properties: (1) they fill the microchannels, preventing flow yet retain mobility; (2) they do not shrink or swell when the running solvent or buffer is changed, so their valving effect works with a variety of solvents; and (3) they insulate electrically when immersed in a conducting fluid.
Property (1) has been well-demonstrated in the prior art using contact lithography techniques or projection lithography for in-situ polymerization in microchannels. Property (2), however, represents a significant challenge since phase-separated porous polymers, in general, will shrink or swell in different solvents in order to reach their minimum potential energy state. This shrinking and swelling can be counteracted by increasing the mechanical strength of the polymer, but cannot be eliminated entirely. Property (3) is also a challenge. While the polymer itself is an insulator and has a high dielectric strength, conduction through the fluid that fills the pores of the porous polymer monolith can allow a current flow that can be a significant fraction of the open-channel conductivity.